In the disclosure of the present invention reference is mostly made to the treatment of diabetes by subcutaneous drug delivery, however, this is only an exemplary use of the present invention.
The most common type of durable drug delivery devices adapted to receive a drug filled cartridge and expel a set dose therefrom are driven by manual means or by a spring energized during dose setting, the cartridge being of the type comprising an axially displaceable piston having an initial proximal position and which is moved distally by a piston rod. Subcutaneous drug delivery takes place via an injection needle arranged in fluid communication with the cartridge. The device may be pen-formed or in the form of a more box-shaped so-called doser. In order to improve convenience, user-friendliness and provide additional features, e.g. detection and storing of expelling data, drug delivery devices have been provided with electrically driven means, typically in the form of an electronically controlled motor driving a piston rod through a gear arrangement, e.g. as shown in U.S. Pat. No. 6,514,230 and US 2011/306927.
Although motorized drug delivery devices facilitate a number of benefits to the customer as many trivial routines can be automated, these devices are often rather large in comparison to conventional mechanical devices and thus they are impractical to carry and store. Addressing this issue WO 03/099357 discloses a pen-formed drug delivery device comprising a drive arrangement including a piston rod in the form of a tubular body that can be moved into a drug cartridge when being advanced in a distal direction, the tubular body defining an internal hollow wherein at least a portion of a motorized drive assembly is arranged when the tubular body is positioned in a retracted proximal position. This configuration for a drive assembly may be termed “motor-in-piston”. Such a drive assembly could also be considered a telescopic drive assembly in which the inner motor drive assembly is the stationary part and the outer tubular body is the moved telescopic part.
Although a motor-in-piston design may result in a compact design, the design involves a number of moving components which interface with multiple other components, all being arranged in a confined space. Such a design will often result in a mechanical design in which the components involved are over-constrained which in turn can lead to malfunction of the device and/or increased production cost and lower production yield.
Having regard to the above, it is an object of the present invention to provide a motorized drug delivery device of the motor-in-piston type as well as components therefor which provide a high degree of reliability in a cost-effective way. It is a further object to provide a general-purpose telescopic drive assembly which is compact in design and provides a high degree of reliability in a cost-effective way.